1. Field of the Invention
The present invention relates to a system and a method for treating exhaust gases from an engine.
2. Background Art
NOx adsorber technology is often times used to reduce the amount of NOx emission (content) in engine exhaust gases. A key component in this technology is the NOx adsorber catalyst, which functions as both an adsorbent and a three-way catalyst. During normal engine operations, the three-way catalyst first oxidizes NOx molecules using the excess oxygen in the engine exhaust, and then stores the oxidized NOx molecules on the adsorbent sites on the catalyst.
To ensure proper operability, the stored NOx must be removed chemically before the adsorbent becomes fully saturated, otherwise the NOx in the exhaust stream will bypass the adsorbent and exit directly to the atmosphere. A substantially oxygen free exhaust stream with adequate CO (carbon monoxide) and HC (hydrocarbon) is often times used to chemically release the stored NOx from the adsorbent sites and convert them to N2 at the three-way catalyst sites. This NOx releasing/converting process is defined as NOx adsorber catalyst regeneration.
To obtain the substantially oxygen free exhaust stream, additional fuel is usually injected into either the engine cylinders or the exhaust pipe, upstream of the catalyst, to consume the oxygen. This additional fuel use typically results in at least an additional 2-6% fuel consumption increase, or a so-called fuel economy penalty, and results in a considerable operation cost for utilizing such an after treatment system.
In order to minimize the fuel economy penalty, the amount of oxygen in the exhaust gases during regeneration should be kept as low as possible. To this end, parallel-arranged dual leg NOx catalyst systems minimize the fuel required by only using a portion of the exhaust gases for catalyst regeneration. These systems have been demonstrated in the laboratory but are typically difficult to install in vehicles because of their space requirements, i.e., they require more space than is typically available.
It would be desirable to provide a system and method for treating exhaust gases from an engine which overcomes at least one of the problems in the prior art.